Modify table: How to change 'Allow Nulls' attribute from not null to allow null

For MySQL, MariaDB

ALTER TABLE [table name] MODIFY COLUMN [column name] [data type] NULL

Use MODIFY COLUMN instead of ALTER COLUMN.

I wrote this so I could edit all tables and columns to null at once:

select 
case
when sc.max_length = '-1' and st.name in ('char','decimal','nvarchar','varchar')
then
'alter table  [' + so.name + '] alter column [' + sc.name + '] ' + st.name + '(MAX) NULL'
when st.name in ('char','decimal','nvarchar','varchar')
then
'alter table  [' + so.name + '] alter column [' + sc.name + '] ' + st.name + '(' + cast(sc.max_length as varchar(4)) + ') NULL'
else
'alter table  [' + so.name + '] alter column [' + sc.name + '] ' + st.name + ' NULL'
end as query
from sys.columns sc
inner join sys.types st on st.system_type_id = sc.system_type_id
inner join sys.objects so on so.object_id = sc.object_id
where so.type = 'U'
and st.name <> 'timestamp'
order by st.name

This is the approach to do this: -

1. Check whether the table or column exists or not.
2. If yes, then alter the column. e.g:-

IF EXISTS (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS WHERE 
            TABLE_CATALOG = 'DBName' AND 
            TABLE_SCHEMA = 'SchemaName' AND
            TABLE_NAME = 'TableName' AND
            COLUMN_NAME = 'ColumnName')
BEGIN
    ALTER TABLE DBName.SchemaName.TableName ALTER COLUMN ColumnName [data type] NULL
END  

If you don't have any schema then delete the schema line because you don't need to give the default schema.

So the simplest way is,

alter table table_name change column_name column_name int(11) NULL;